Apparatus that include compartments for High Vacuum (HV) environments are well-known and widely used in, for example, the manufacture of integrated circuits (ICs), where the high vacuum serves to minimize the presence of contaminating particles or species. Due to interaction of these particles or species with, for example, a projection beam in the apparatus, optical components and/or a substrate which is to be manufactured into an IC, these particles may have a detrimental effect on the accuracy of the apparatus.
The term “High Vacuum environment” encompasses a gaseous environment with a pressure of less than 10 hPa. Also, environments usually referred to as Ultra High Vacuum (UHV) or even higher are covered by the term “High Vacuum environment” or “High Vacuum”.
A compartment in which at least temporarily a high vacuum is to be maintained is generally constructed from construction elements of stainless steel, or in exceptional cases aluminum. It is common practice to use stainless steel for constructing a compartment for HV, as this is generally believed to provide a low cost material which in use still meets predetermined requirements. One of the main advantages of stainless steel is its high resistance to corrosion. In High Vacuum environments, corrosion is to be avoided as, especially with iron-based materials, corrosion is typically accompanied by the formation of a rough and very porous surface structure (‘rust’), which allows for an enormously large surface to which molecules can adsorb. In the porous surface structure, volumes are formed from which it may be difficult or impossible to remove the gasses by pumping within a reasonable time (often called ‘virtual leaks’ in vacuum technology). Facing a High Vacuum environment, these sticky molecules may at some stage be released by the surface into the space which is meant to be held under vacuum conditions, which counteracts the maintenance of the vacuum. Hence, stainless steel, which has a high resistance to corrosion and thus a high resistance to the formation of the porous surface structures, has proved to be a very useful material for constructing compartments in which a high vacuum is to be maintained. Stainless steel has also proved to be a useful material for at least partly constructing components which are to be placed in such a compartment.
Even though stainless steel is one of the least expensive materials known today that is suitable for constructing High Vacuum compartments, stainless steel still remains a relatively expensive material. The suitability of stainless steel for use in environments which are to be maintained under High Vacuum is in this context referred to as vacuum compatibility of stainless steel.
Although in the remaining part, reference may only be made to constructing compartments in which a high vacuum is to be maintained, it is understood that such reference also applies to constructing components which are to be placed in such a compartment.